Aluminum cementation process



United States Patent 3,415,676 ALUMINUM CEMENTATION PROCESS Sunao Nishi and Shojiro Ikeda, Nagasaki-shi, Japan, assignors to Shinto Kogyo Kabushiki Kaisha, Nagasakishi, Nagasaki-ken, Japan, a joint-stock company No Drawing. Filed Aug. 13, 1965, Ser. No. 479,624 Claims priority, application Japan, Sept. 14, 1964, 39/52,215 2 Claims. (Cl. 117107.2)

ABSTRACT OF THE DISCLOSURE When diffusing Al into the outer layer of a metal article (cementation), the following steps have been found to insure economic utilization of Al. An open or nonsealed vessel is filled with a penetrant which consists of a mixture of powder selected from a group consisting of Fe powder or Fe-Al alloy powder containing a lower concentration of Al than the total penetrant, as well as Al powder having a particle size greater than 300 mesh and a sufficient quantity of cementation accelerator. A metal article is then embedded in the penetrant and the vessel is heated to approximately 1000 C. to cause diifusion of the Al into the metal article.

This invention relates to a process for cementation with aluminum by using powder materials and is characterized by the use of a penetrant consisting of a mixture of Fe powder or a Fe-Al alloy powder of low Al content and Al powder and the use of a non-sealed or open vessel. Throughout this specification the term Fe-Al alloy powder of low aluminum content (or concentration) does not mean an alloy powder of a specific proportion but means an alloy powder containing a lower concentration of aluminum than the penetrant itself. It is also to be understood that the Fe powder and the Al powder may contain impurities not adversely affecting the cementation process. 7

A number of different processes for aluminum cementation have been proposed in the past. For example, in one method a mixture of A1 0 and Al powder is utilized, and in another method a powder of alloy containing equal quantities of Fe and Al is utilized as the penetrant. However, the former method requires complicated process steps since according to this process a cementation layer is formed by rotating a vessel and heating the article while H gas is introduced into the vessel to fix the aluminum and then reheating the article, moreover, this process is accompanied by danger and results in the deformation of articles of large size or of complicated configuration, so that it is difficult to always form cementation layers of the desired property. Further, by the latter process it is necessary to use a large quantity of excess aluminum in order to form cementation layers of practical value. For example, in an operation of a scale requiring 7000 kilogrammes of the penetrant per batch, it is necessary to use 2100 kilogrammes of an alloy powder containing equal quantities of Fe and Al in order to permeate 600 kilogrammes of aluminum in the article to be treated. This means that 450 kilogrammes out of 1050 kilogrammes of aluminum are wasted, similarly, 2800 kilogrammes of alloy containing equal quantities of Fe and Al are required to cause cementation of 800 kilogrammes of aluminum, which results in the waste of 600 kilogrammes of aluminum.

Accordingly, it is the principal object of this invention to prepare aluminum cementation layers having the same equality as those prepared by conventional methods but by a simple operation without accompanying waste of the raw material.

A further object of this invention is to accomplish aluminum cementation without the necessity of using H gas, a rotary container, or a reheating step for forming the cementation layer.

Briefly stated, the process for aluminum cementation of this invention is characterized by the steps of embedding an article to be treated in a penetrant consisting of a mixture of Fe powder or Fe-Al alloy powder of low aluminum concentration, aluminum powder having a grain size larger than 300 meshes, and a small quantity of cementation accelerator in a non-sealed vessel and causing diffusion by heating.

Heretobefore, it has been believed that aluminum powder is highly inflammable and readily ignites when heated in air, so that aluminum has not been utilized as a penetrant for aluminum cementation processes on a commercial scale. When used, it was necessary to use a reaction device of complicated construction in order to prevent ignition of the aluminum.

However, as a result of an extensive investigation, we have found that aluminum powder does not necessarily ignite readily in air when heated to a temperature up to 1000 C. and that this property is dependent upon the grain size of the aluminum powder. For example, when samples of aluminum powder of varying grain size which were piled in piles of cone shape were placed in a furnace maintained at a temperature of from 700 to 1000 C. and quickly heated, it was found that only the samples having a grain size less than about 300 meshes were ignited, and almost none of the samples of aluminum powder having a larger grain size than 250 meshes were ignited. As an illustrative example, when an aluminum powder containing a random mixture of grains of various size in a range of from 40 to 250 meshes is placed in. a furnace maintained at 1000 C. and then heated rapidly, at portions of the surface layer of the aluminum. powder, several adjacent particles fuse together to form substantially perfect spheres having diameters of from a few to several millimetres, but the remaining major portion does not fuse or ignite and exhibits a powdery appearance like that of the original powder of changed colour. When cross-sections of the powder after heating are examined under a microscope, it is observed that each particle consists of an independent, perfect sphere of metallic aluminum powder, although the grain sizes of different particle-s vary depending upon the original grain size. It is believed that this is caused by the fact that the surface of each grain is oxidized to form a shell before it is heated to its melting temperature, so that the particle itself becomes molten in the interior, thus forming a sphere owing to its surface tension, but adjacent particles do not fuse together, and that the powder does not ignite since shells formed on individual grains prevent 0 from coming into contact with the aluminum. Fusion of a portion of the upper layer may be attributed to the fact that grains in such a portion are instantly melted by the direct heating of high temperature, so that adjacent grains fuse together before the shells are formed thereon. When a mixture consisting of Fe powder of a grain size of from 60 to meshes which has been oxidized to an extent such that the surfaces of the grains are covered by red rust and aluminum powder of a grain size of from 20 to 300 meshes is placed in a hermetically sealed vessel and then heated slowly, the powder invari' ably burns explosively when a temperature of 700 C. is reached, and all of the aluminum powder is burned. On the other hand, iron powder with a clean surface does not burn even when it is placed in a furnace maintained at 1000 C. and quickly heated therein, and the aluminum permeates into the iron and is preserved therein.

From the above investigation it was concluded that aluminum powder has a tendency to ignite when it coexists with a metal oxide which can be readily reduced, such as iron oxide and the like, but, in counterdistinction, is difficult to burn in air. This invention contemplates utilization of aluminum powder as an efficient penetrant without utilizing a closed vessel or a reducing gas.

A preferred method of preparing the Fe-Al alloy powder is as follows. 70 parts of ground Fe powder having a grain size of 3080 meshes and 30 parts of Al powder having a grain size of 20-300 meshes are placed in a vessel. Upon heating the mixture Al begins to deposit on the surface of Fe powder at 500 C., the deposited quantity increasing with temperature and time. At 660 C. all of the aluminum is deposited as thin layers on the surface of Fe powder, whereby no free aluminum remains. Thus, at this stage it is possible to observe the formation of thin layers of Fe-Al alloy. By continuing the heating, the concentration of aluminum on the surface of the Fe powder decreases gradually but the thickness of the Fe-Al alloy increases correspondingly. Finally, an equilibrium condition is attained wherein each iron particle is covered into a Fe-Al alloy containing approximately 30% of aluminum. When heating is continued after this equilibrium condition has been attained, there is no change in the structure and hardness.

When an arbitrary quantity of Fe powder is mixed with said Fe-Al alloy powder in equilibrium condition, and then the mixture is heated in a vessel, said equilibrium condition is destroyed by the presence of Fe powder, so that the aluminum in the Fe-Al alloy leaches out and penetrates into adjacent particles of Fe powder. Thus, when the mixture is heated sufficiently, a new equilibrium condition is established to produce a new Fe-Al alloy of lower aluminum concentration and pure Fe powder no longer remains.

Further, when an arbitrary quantity of aluminum is mixed with the Fe-Al alloy powder in equilibrium condition, and the mixture is heated to a suitable temperature, aluminum disappears by permeating into the Fe-Al alloy powder thereby to form a new Fe-Al alloy powder having higher concentration of aluminum than the original alloy to establish a new equilibrium condition.

Thus, aluminum has the unique property of flowing from the higher aluminum concentration area to the lower concentration area when heated as an alloy or mixture with iron. In this case, the iron serves as carrier for the aluminum and assists free flow thereof and functions to preserve the aluminum in a manner similar to that of a sponge absorbing and preserving water. If aluminum powder is loaded in a vessel and heated to a temperature above its melting point, strong shells will not be formed owing to shortage of air, and the whole mass of the powder will be fused or sintered into a unitary body. Since the practical and economical temperature for aluminum cementation is approximately 1000 C., it is necessary to add a substance effective for preventing fusing and sintering at this temperature. Refractories such as A1 and SiO are quite suitable for this purpose, and, as stated before, while a method of utilizing A1 0 is well known, the melting points of these refractories are very high, i.e., about 2000 C., which is amply high with respect to the temperature for cementation of about 1000 C. As is evident from phase diagrams, the melting points of Fe powder and Fe-Al alloy powder having low aluminum concentration are far lower than those of said refractories, but Fe or Fe-Al alloy does not melt at a temperature near 1000" C. so that they can completely prevent aluminum powder from sintering.

As has been described hereinabove, aluminum powder does not burn at high temperatures even when it is maintained in contact with air and can freely flow through the iron, while the iron functions not only to afford a flow path for the aluminum but also as a carrier for preserving the aluminum. In addition, the iron powder as well as the Fe-Al alloy powder effectively prevents sintering of the aluminum. This invention is based on the effective utilization of these properties.

In carrying out this invention, aluminum of a quantity desirable for penetration into an article to be treated is incorporated into an Fe-Al alloy powder of low aluminum concentration. For example, when it is desired to cause cementation with 600 kg. of aluminum as in the foregoing example, 600 kg. of aluminum is mixed with a small quantity of cementation accelerator, such as ammonium chloride, to produce a penetrant. Where it is desired to cause cementation with 800 kg of aluminum, 800 kg. of aluminum is used. The article to be treated is embedded in the penetrant contained in a non-sealed vessel having small gas vent holes. Where a plurality of articles are to be treated simultaneously, they must be embedded with a proper spacing between them. After heating at a suitable temperature for a suitable interval of time, the aluminum in the penetrant penetrates into the surface of the article which is then cooled and removed from the vessel when the desired quantity, for example, 600 kg. or 800 kg., has been caused to penetrate. After treatment, the Fe-Al alloy powder contains the same quantity of aluminum as the Fe-Al alloy of low aluminum concentration prior to incorporation of aluminum powder. Thus, the concentration of aluminum of the alloys does not vary before and after treatment, so that it appears as if only the aluminum so incorporated is subjected to the cementation.

The process of this heat treatment will now be considered in greater detail. When the temperature is increated to approximately C. the cementation accelerator decomposes to evolve gas. When the gas is evolved in large quantity, a portion thereof is exhausted together with some of the air remaining in the vessel to the outside of the reaction vessel through said gas vent holes to decrease further the quantity of air in the vessel. Since the interior of the vessel is always maintained at a positive pressure to prevent infiltration of the atmosphere, it is not necessary to provide valves or other means for venting. If the vessel were to be sealed hermetically without providing vents, there would be the danger of the vessel bursting owing to the internal gas pressure.

When the temperature in the vessel reaches 500 C., the aluminum begins to deposit on the surface of the Fe-Al alloy of low aluminum concentration which is in contact with the aluminum powder. At 660 C. all of the aluminum is deposited, and concurrently with its deposition, the aluminum begins to diffuse toward the interior. It is to be noted that only a portion of the aluminum powder deposits directly on the surface of the article to be treated, and the greater part thereof deposits on the surface of the Fe-Al alloy powder of low aluminum concentration. However, as the gradient of concentration of aluminum directly deposited on the surface of the article to be treated is very steep, that is, almost vertical, the aluminum will rapidly diffuse toward the interior. As a result, the concentration of aluminum on the upper surface is decreased below that of the Fe-Al alloy powder, in which the article is embedded. In the meantime, in each particle of Fe-Al alloy, the aluminum deposited on the surface is continually diffusing and penetrating toward the interior to approach equilibrium, but upon decrease in the aluminum concentration of the surface of the article to be treated, the aluminum contained in the particles which are in contact with said surface immediately begin to transfer to the surface of the article, thus decreasing the aluminum concentration of said particles. Then the aluminum contained in particles remote from said surface transfer to the particles situated close to said surface. In this manner, the aluminum contained in particles which are positioned remotely from the surface of the article is transferred to the surface via intermediate particles to penetrate into the article. Thus, although it appears that only the aluminum supplemented forms the cementation layer, actually, the aluminum retained in the Fe-Al alloy of low aluminum concentration also contributes to form the cementation layer, and the decrease in the aluminum content of the Fe-Al alloy is automati' cally supplemented.

Where the article is treated with a mixture of iron powder and aluminum powder, it is necessary only in the first operation to add aluminum of the quantity required for forming a Fe-Al alloy having the same alumiv num concentration as that of the cementation layer of the article in addition to the aluminum to be introduced into the article. This is because aluminum has the inherent property of flowing from the higher concentration area to the lower concentration area as has been pointed out above, so that if the aluminum concentration of the penetrant surrounding the article were low, the aluminum concentration of the cementation layer would also be lower than the desired value, thus failing to produce a satisfactory product. However, by heating the product, the desired cementation layer can be produced by the same procedure as that mentioned above. It may appear at first thought that this process requires a large quantity of aluminum and wastes a substantial quantity thereof, but during the second operation and thereafter, the iron is converted into a Fe-Al powder, and the aluminum preserved in this form is utilized effectively during the second and succeeding operations, whereby no waste or useless hoarding whatsoever is involved.

Thus, this invention provides a novel method for aluminum cementation which eliminates all disadvantages of the prior art methods such as the use of complicated and dangerous operations involving passage of H gas, rotation of the reaction vessel, and reheating for forming the cementation layer, troubles in producing an alloy containing iron and aluminum in equal quantities, large waste of aluminum, and the like and which can produce products of good quality by a simple and economic operation.

As illustrative examples the following preferred examples are given, but it should be understood that this invention is not limited thereto.

EXAMPLE 1 Three samples of steel pieces were embedded in a penetrant consisting of a mixture of 65 parts of iron powder, 35 parts of aluminum powder, and 1 part of ammonium chloride and contained in a vessel comprising a steel tube provided with a plurality of small vent holes. These materials were maintained at about 1000" C. for 10 hours and then cooled to produce cementation layers containing 92.3 mg./cm. 94.6 mg./cm. of penetrating aluminum and having a thickness of 0.64 mm.0.65 mm.

The heat resistant property of the samples was com- The same procedure as in Example 1 was repeated except that the penetrant used consisted of 88 parts of Fe-Al alloy powder having an aluminum concentration of 29.23%, 12 parts of aluminum powder, and 1 part of ammonium chloride to produce a cementation layer containing 98.0 mg./cm. 101.4 mg./cm. of penetrating aluminum and a thickness of 0.65 mm.0.66 mm. After treatment the Fe-Al alloy showed an aluminum concentration of 29.74%, so that it was possible to use the alloy in the next operation to form a cementation layer containing substantially the same quantity of aluminum as the first operation.

The aluminum powder utilized in the foregoing examples contained 98.7% of metallic aluminum and contained random proportions of particles of from 20 to 300 meshes.

What we claim is:

1. An aluminum cementation process which comprises the steps of placing a penetrant in an. open vessel, said penetrant consisting of a mixture of powder selected from a group consisting of Fe powder or Fe-Al alloy powder containing a lower concentration of Al than the total penetrant, as well as Al powder having a particle size greater than 300 mesh and a sufficient quantity of a cementation accelerator; embedding a metal article to be treated in said penetrant; and heating said vessel and contents to cause the aluminum contained] therein to diffuse into said article.

2. An aluminum cementation process as claimed in claim 1 wherein the cementation accelerator is ammonium chloride, the metal article is steel and the vessel is heated to approximately 1000" C.

References Cited UNITED STATES PATENTS 2,887,420 5/ 1959 Llewelyn et al. 3,096,160 7/1963 Puyear. 3,108,013 10/1963 Chao et al. 3,257,230 6/1966 Wachtell et a1.

ALFRED L. LEAVITT, Primary Examiner.

A. G. GOLIAN, Assistant Examiner.

US. Cl. X.R. 117130 

